1. Field of the Invention
The present invention relates to a pressure sensor using a pressure sensing element and a diaphragm. Especially, the present invention relates to a technique for reducing an error in a pressure measurement value of a pressure sensor, which is accompanied by temperature change and caused by a combination of different kinds of materials; and an error in a pressure measurement value, which is caused by flexure of a diaphragm due to gravity force.
2. Invention of Related Art
Pressure sensors that include a piezoelectric resonator element as a pressure sensing element are known as a water pressure gauge, an air gauge, a differential pressure gauge, or the like. The piezoelectric resonator element includes, for instance, a planar piezoelectric substrate on which an electrode pattern is formed, and a detection axis is set to be a direction in which force is detected. When pressure is applied in the direction of the detection axis, a resonance frequency of the piezoelectric resonator changes and the pressure is detected from the change of the resonance frequency. Japanese Unexamined Patent Application Publication No. 2007-57395 discloses a pressure sensor according to a first related art. FIG. 21 shows the pressure sensor according to the first related art. This pressure sensor 201 according to the first related art includes: an airtight case 202; a first pressure input orifice 203a; a second pressure input orifice 204a; a first bellows 210; a second bellows 211; a resonator element adhesive pedestal 215; a piezoelectric resonator element 220; a piezoelectric reinforcing plate 221; and an oscillation circuit 230. The inside of the airtight case 202 is vacuumed or the airtight case 202 contains inert atmosphere therein. The first and second pressure input orifices 203a and 204a are respectively formed on a first wall 203 and a second wall 204, which are opposed to each other, of the airtight case 202 in a manner to respectively penetrate the walls 203 and 204. The first bellows 210 has a cylindrical shape of which an opening at one end is fixed to the first wall 203 and has an axis hole communicating with the first pressure input orifice 203a. The second bellows 211 has a cylindrical shape of which an opening at one end is fixed to the second wall 204, has an axis hole communicating with the second pressure input orifice 204a, and is disposed in series with the first bellows 210. The resonator element adhesive pedestal 215 is disposed and fixed between the other ends 210a and 211a of the first and second bellows 210 and 211. The piezoelectric resonator element 220 has a thin plate shape and is supported by the resonator element adhesive pedestal 215. The piezoelectric reinforcing plate 221 is disposed to be opposed to the piezoelectric resonator element 220 with the second bellows 211 therebetween. The oscillation circuit 230 communicates with an electrode pattern formed on the piezoelectric resonator element. The piezoelectric resonator element 220 is fixed to the second wall 204 at its one end and fixed to the resonator element adhesive pedestal 215 at the other end. The piezoelectric reinforcing plate 221 is fixed to the second wall 204 and the resonator element adhesive pedestal 215 at its respective ends. The resonator element adhesive pedestal 215 and an inner wall of the airtight case 202 are fixed to each other by a reinforcing plate spring so as to increase durability against impact in X-axis direction.
The piezoelectric resonator element 220 has a structure in which an electrode is formed on a quartz crystal substrate, for example. The resonator element adhesive pedestal 215 includes a base portion 215a which is fixed in a manner to be sandwiched by the other ends 210a and 211a of the bellows 210 and 211 and a supporting piece 215b protruding from a circumference of the base portion 215a toward the second wall. The other ends of the piezoelectric resonator element 220 and the piezoelectric reinforcing plate 221 are both connected to the supporting piece 215b. 
The pressure input orifices 203a and 204a are respectively communicated with the axis holes of the inside of the bellows 210 and 211, while the axis holes in the bellows are maintained at a state that they are not communicated with each other due to the base portion 215a of the resonator element adhesive pedestal 215. Therefore, a position of the resonator element adhesive pedestal 215 is moved forward and backward in an axis direction of the bellows due to expansion and contraction of the bellows generated by pressure difference between pressure P1 and pressure P2 which are respectively inputted from the pressure input orifices 203a and 204a. The piezoelectric resonator element 220 fixed to the resonator element adhesive pedestal 215 at its one end and fixed to the second wall 204 at the other end receives mechanical stress in the axis direction due to pressure transmitted from the resonator element adhesive pedestal 215 so as to deform, changing natural resonance frequency. That is, an excitation electrode is energized in a state that the oscillation circuit 230, which is disposed on an appropriate position of the airtight case 202 in an airtight state, and the excitation electrode, which constitutes the piezoelectric resonator element 220 and is formed on a piezoelectric substrate, are coupled so as to vibrate the piezoelectric substrate, and pressure P1 and pressure P2 are calculated based on an output frequency at this time.
According to the pressure sensor 201 of the first related art, when pressure P1 is inputted into the first pressure input orifice 203a, force corresponding to the pressure is applied to the piezoelectric resonator element 220 and the piezoelectric reinforcing plate 221. Because of an existence of the piezoelectric reinforcing plate 221, only force in a longitudinal direction (Y-axis direction in the drawing in a case of a quartz crystal resonator element) is applied to the piezoelectric resonator element 220, and therefore primary pressure-frequency property of the piezoelectric resonator element exhibits a quadratic curve. Accordingly, the resonance frequency of the piezoelectric resonator element 220 changes in a linear fashion while corresponding to pressure P1, being able to obtain the pressure sensor 201 with high accuracy.
Japanese Unexamined Patent Application Publication No. 2002-214058 discloses a pressure sensor 300 according to a second related art. FIG. 22 shows the pressure sensor 300 according to the second related art. The pressure sensor 300 according to the second related art has a structure in which a silicon structure 305 including a diaphragm 309 is bonded to a base body 306 provided with an electrode 307 and a dielectric layer 308 in such a manner that the diaphragm 309 is opposed to the electrode 307 and a gap 310 is formed between the diaphragm 309 and the dielectric layer 308. The electrode 307 is composed of a metal thin film. The dielectric layer 308 is formed to cover the electrode 307. The diaphragm 309 can deform in response to pressure and has conductivity. With such the structure, the pressure can be detected by detecting change in capacitance accompanied by change of a contacting area, in which the diaphragm 309 contacts with the dielectric layer 308 by receiving pressure.
However, in the invention of Japanese Unexamined Patent Application Publication No. 2007-57395, it is difficult to match linear expansion coefficients of the piezoelectric resonator element 220 and the airtight case 202. Accordingly, stress applied on the piezoelectric resonator changes when a temperature changes, and this stress change due to the temperature change brings an error in a pressure measurement value. The bellows is used so as the pressure measurement value to be hardly influenced by the linear expansion coefficient in Japanese Unexamined Patent Application Publication No. 2007-57395, but the bellows can not completely eliminate the influence of the linear expansion coefficient.
Further, in the invention of Japanese Unexamined Patent Application Publication No. 2002-214058, the diaphragm 309 receives force in a direction approaching the base body 306 due to gravity of the earth. In addition, if the pressure sensor 300 is disposed in an inverted manner, the diaphragm 300 receives force in a direction leaving from the base body 306 due to the gravity of the earth. Accordingly, the pressure sensor 300 is originally intended to detect pressure of gas or liquid, but an error in a pressure measurement value arises depending on its disposed posture.